The present invention relates to a circularly polarized cross dipole antenna which is favorably used as a mobile communication antenna for a GPS wave receiving system, a transmitting/receiving system of a satellite communications cellular phone, and the like.
FIGS. 10A and 10B are illustrations for describing an overview of a prior art circularly polarized cross dipole antenna. FIG. 10A illustrates a dipole antenna, while FIG. 10B does a cross dipole antenna. The dipole antenna shown in FIG. 10A is assembled by forming a single dipole antenna element 101 on a ground plate 100, whereas the cross dipole antenna shown in FIG. 10B is assembled by forming a pair of dipole antennas 101 and 102 on the ground plate 100 so as to cross each other. The cross dipole antenna excites a circularly polarized wave by shifting its phase 90 degrees.
An axial ratio characteristic is important to an antenna for exciting a circularly polarized wave. In the cross dipole antenna illustrated in FIG. 10B, the axial ratio characteristic of each of the dipole antenna elements 101 and 102 crossing each other is a problem. The axial ratio characteristic becomes good when a gain characteristic of E plane (where an electric field is generated) in each of the dipole antenna elements 101 and 102 is equal to that of H plane (where a magnetic field is generated) therein. When these gain characteristics differ from each other, the axial ratio characteristic becomes worse by an amount corresponding to the difference.
FIG. 11 is a chart of the comparison of a gain characteristic of E plane (C1 indicated by the solid line) and that of H plane (C2 indicated by the broken line) in the single dipole antenna element 101 shown in FIG. 10A. It is seen from FIG. 11 that the gain characteristics C1 and C2 are different very widely.
If a cross dipole antenna is assembled by simply crossing two dipole antenna elements having the above characteristics, an axial ratio of them is satisfactory in the vicinity of 0.degree. but it is unsatisfactory at the other angles. It is thus difficult to obtain a circularly polarized cross dipole antenna having a wide-angle axial ratio characteristic even though it is assembled by simply combining two dipole antenna elements having a conventional structure.